Light on interaction with matter undergoes scattering. Most of the incident photons are elastically scattered (Rayleigh scattering) while one out of ten million are inelastically scattered. Inelastic scattering of photon is termed as Raman scattering. Raman scattering occurs due the change in the polaraizability of a molecule thereby leading to a change in its vibrational state. This results in the emission of a photon having energy lower (Stokes Raman) or higher (anti-Stokes Raman) than that of the incident photon depending upon the initial vibrational state of the molecule. The shift in Raman frequency provides chemical and structural information. However, Raman scattering is relatively weak leading to low detection sensitivity and in consequence, it's difficult to measure low concentrations and weak Raman scatterer. This can be overcome using Resonance Raman (RR) technique wherein the wavelength of the exciting photon lies within the electronic absorption of the molecular system. Under this condition, the Raman signals can be enhanced by a factor of 102 to 104. But, in case of fluorescent molecules or presence of fluorescent impurities, the strong fluorescence signal masks the weak Raman signals.
However, owing to its low scattering cross-section one needs to accumulate for longer time to obtain good signal. Thus, the routine application of spontaneous Raman spectroscopy for the vibrational structure determination is limited to non-fluorescent materials with relatively strong Raman cross-section.
Advanced Raman spectroscopic techniques, such as coherent anti-Stokes Raman scattering (CARS)[1], picosecond (ps) Kerr-gate[2], stimulated Raman scattering (SRS)[3-11], etc. have been developed to overcome these problem. All these processes are characterized by the third order nonlinear susceptibility (χ[3]) of the system. Both CARS and SRS involve a four wave mixing process providing the signal. While in ps Kerr gate spectroscopy, a nonlinear phenomenon, Kerr effect, is used as a gate for the detection of the instantaneous Raman scattering signals before being overwhelmed by the fluorescence signal. Kerr effect occurs due to a nonlinear change in the refractive index of a material in the presence of a short laser pulse (gating pulse). These methods provide Raman spectrum with a good signal to noise ratio and efficient fluorescence rejection compared to conventional Raman spectroscopy. Yet, these methods suffer from some difficulties.
For example, in Kerr-gate technique[2], fluorescence cannot be completely eliminated and background signal from long-lived samples add to the noise. While CARS efficiency is much greater than that of spontaneous Raman scattering, its sensitivity is limited by structureless background arising from a non-resonant component of the third order susceptibility. In dilute solutions, this background signal is due to solvent molecules. This non-resonant background leads to a distorted dispersed signal. In addition, a CARS signal is directly proportional to the square of the spontaneous Raman spectrum. This has effect of enhancing the strong features at the expense of the weaker ones. Further, CARS experiment requires the interacting laser beams to follow a specific phase matching condition, rendering the technique extremely sensitive to alignment of laser beams and the angles of interaction.
All of the patents related to analytical applications of Raman spectroscopy or its derivations, used for structure elucidation, discuss only the positive signal (GAIN) in Raman Spectroscopy. The instant invention is very unique in observation of the signals as LOSS with more intensity than the GAIN (positive signal).
There are patents related to LOSS observation and utility in optical communications by fibers, which are related to modulation of ACOUSTIC FREQUENCY signals, unlike what the present invention where modulation of LIGHT FREQUENCY leads to vibrational structural information of system under study. This is a significant difference, since the technology for generation and detection of both, acoustic and light signals are entirely different.